Network Working Group M. Tuexen
Internet-Draft I. Ruengeler
Updates: 4960 (if approved) Muenster Univ. of Appl. Sciences
Intended status: Standards Track R. R. Stewart
Expires: October 10, 2013 Adara Networks
April 08, 2013
SACK-IMMEDIATELY Extension for the Stream Control Transmission Protocol
draft-ietf-tsvwg-sctp-sack-immediately-03.txt
Abstract
This document updates RFC 4960 by defining a method for the sender of
a DATA chunk to indicate that the corresponding SACK chunk should be
sent back immediately and not be delayed. It is done by specifying a
bit in the DATA chunk header, called the I-bit, which can get set
either by the SCTP implementation or by the application using an SCTP
stack. Since unknown flags in chunk headers are ignored by SCTP
implementations, this extension does not introduce any
interoperability problems.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on October 10, 2013.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. The I-bit in the DATA Chunk Header
The following Figure 1 shows the extended DATA chunk.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0 | Res |I|U|B|E| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TSN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Stream Identifier | Stream Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Protocol Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ \
/ User Data /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Extended DATA chunk format
The only difference between the DATA chunk in Figure 1 and the DATA
chunk defined in [RFC4960] is the addition of the I-bit in the flags
field of the DATA chunk header.
This bit was Reserved in [RFC4960]. [RFC4960] specified that this
bit should be set to 0 by the sender and ignored by the receiver.
4. Use Cases
The setting of the I-bit can either be triggered by the application
using SCTP or by the SCTP stack itself. The following two
subsections provide a non-exhaustive list of examples.
4.1. Triggering at the Application Level
One example of a situation in which it may be desirable for an
application to trigger setting of the I-bit involves the
SCTP_SENDER_DRY_EVENT in the SCTP socket API [RFC6458]. Upper layers
of SCTP using the socket API as defined in [RFC6458] may subscribe to
the SCTP_SENDER_DRY_EVENT for getting a notification as soon as no
user data is outstanding anymore. To avoid an unnecessary delay
while waiting for such an event, the application can request the
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Internet-Draft SACK-IMMEDIATELY April 2013
setting of the I-Bit when sending the last user message before
waiting for the event. This results in setting the I-bit of the last
DATA chunk corresponding to the user message and is possible using
the extension of the socket API described in Section 7.
4.2. Triggering at the SCTP Level
There are also situations in which the SCTP implementation can set
the I-bit without interacting with the upper layer.
If the association is in the SHUTDOWN-PENDING state, setting the
I-bit reduces the number of simultaneous associations for a busy
server handling short living associations.
Another case is where the sending of a DATA chunk fills the
congestion or receiver window. Setting the I-bit in these cases
improves the throughput of the transfer.
If an SCTP association supports the SCTP Stream Reconfiguration
extension defined in [RFC6525], the performance can be improved by
setting the I-bit when there are pending reconfiguration requests
that require that there be no outstanding DATA chunks.
5. Procedures5.1. Sender Side Considerations
Whenever the sender of a DATA chunk can benefit from the
corresponding SACK chunk being sent back without delay, the sender
MAY set the I-bit in the DATA chunk header. Please note that it is
irrelevant to the receiver why the sender has set the I-bit.
Reasons for setting the I-bit include, but are not limited to, the
following (see Section 4 for the benefits):
o The application requests to set the I-bit of the last DATA chunk
of a user message when providing the user message to the SCTP
implementation (see Section 7).
o The sender is in the SHUTDOWN-PENDING state.
o The sending of a DATA chunk fills the congestion or receiver
window.
o The sending of an Outgoing SSN Reset Request Parameter or an SSN/
TSN Reset Request Parameter is pending, if the association
supports the Stream Reconfiguration extension defined in
[RFC6525].
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Internet-Draft SACK-IMMEDIATELY April 20135.2. Receiver Side Considerations
On reception of an SCTP packet containing a DATA chunk with the I-bit
set, the receiver SHOULD NOT delay the sending of the corresponding
SACK chunk, i.e., the receiver SHOULD immediately respond with the
corresponding SACK chunk.
6. Interoperability Considerations
According to [RFC4960] the receiver of a DATA chunk with the I-bit
set should ignore this bit when it does not support the extension
described in this document. Since the sender of the DATA chunk is
able to handle this case, there is no requirement for negotiating the
support of the feature described in this document.
7. Socket API Considerations
This section describes how the socket API defined in [RFC6458] is
extended to provide a way for the application to set the I-bit.
Please note that this section is informational only.
A socket API implementation based on [RFC6458] needs to be extended
to allow the application to set the I-bit of the last DATA chunk when
sending each user message.
This can be done by setting a flag called SCTP_SACK_IMMEDIATELY in
the snd_flags field of the struct sctp_sndinfo structure when using
sctp_sendv() or sendmsg(). If the deprecated struct sctp_sndrcvinfo
structure is used instead when calling sctp_send(), sctp_sendx(), or
sendmsg(), the SCTP_SACK_IMMEDIATELY flag can be set in the
sinfo_flags field. When using the deprecated function sctp_sendmsg()
the SCTP_SACK_IMMEDIATELY flag can be in the flags parameter.
8. IANA Considerations
[NOTE to RFC-Editor:
"RFCXXXX" is to be replaced by the RFC number you assign this
document.
]
Following the chunk flag registration procedure defined in [RFC6096],
IANA should register a new bit, the I-bit, for the DATA chunk. The
suggested value is 0x08 and the reference should be RFCXXXX.
This requires an update of the "DATA Chunk Flags" registry for SCTP:
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